The invention relates to a low light level television system having a first channel which detects the nocturnal light reflected by objects in the visible spectrum and in the near infrared (400 nm to approximately 1000 nm) and a second channel which detects another part of the spectrum, each channel including a low light level picture pick-up or detector tube and an input lens adapted to the range of the detected wavelength, the sensitivity range of the first channel being determined by the sensitivity of the photocathode of the detector tube, the signals of the two channels being combined to produce an image on a television monitor.
An invention of this type is disclosed in the document FR 2168427. Such a system includes two channels which operate over different wavelength ranges: one channel extends from 400 nm to 900 nm and the other channel in the thermal infrared (transmission). The two channels are each provided with a lens adapted to the transferred wavelength and two detectors adapted to the wavelength ranges. The signals supplied by each channel are combined in a cathode-ray tube.
In the case of low light level imaging there is a need for devices of small bulk, of low cost, and which are portable, for manufacture for uses by the general public such as the monitoring of premises. The thermal infrared detectors are too expensive for this type of usage and other solutions must be looked for. When the thermal infrared detector is omitted, an apparatus is obtained which operates with one colour only (grey level to show the objects and their contour). In such a system, to obtain the best possible degree of detection and to enable the recovery of an image which is perceptible even at very low light levels (E&lt;10.sup.-5 lux) corresponding to the darkest nights, photocathodes are used which have the highest spectral sensitivity range, especially towards the near infrared, for example photocathodes S25 or photocathodes made of GaAs and having the high sensitivity. Actually, in the near infrared the radiation of objects is better than in the visible range. In spite of that, the detection of certain objects might become difficult by lack of contrast between certain materials. In the case, for example, of vegetation and concrete, their coefficients of reflection as a function of the wavelength are different. The vegetation reflects approximately 10% in the visible and approximately 60% in the near infrared, whilst the concrete reflects approximately 30% along the overall spectrum. There is consequently an inversion of contrast towards the centre of the spectral domain of the sensitivity of photocathodes S25 near 700 nm approximately. With such a photocathode, the vegetation and the concrete give the same photocathode current, so the same luminance on the display screen: the contrast between these two materials will seem to be zero or very weak.
A purely optical approach to recover the intensified and coloured images, has been proposed by H. MULDER in "Advances in Electronics and Electronics physics--6.sup.th symposium on photoelectronic image devices--London 1976 Vol. 40A page 33". The article describes an image intensifying system with direct vision. The spectrum is separated into two parts, an infrared part whose wavelengths are significantly higher than approximately 700 nm and a visible part in which the wavelengths are significantly less than 700 nm. Each part of the spectrum is separately intensified and is recovered into specific colours, for example green for the near infrared, and red for the visible light. Thus, with such a dichrome system the vegetation transmitting more in the infrared will be seen as green whilst the concrete will be seen as brown (a mixture of green and red). The contrast between these two materials will then be provided by a colour difference and these differences are then identified.
Such a dichrome system utilizes one single tube and separates the spectrum into two distinct parts by means of a dichroic mirror. Such a dichrome system has the advantage that each half of the tube receives only a portion of the phototonic spectrum, so less photones than in a monochrome system, which results in a relative increase in noise, consequently in a deterioration of said detectability of each channel.